1. Field of the Invention
The invention relates to a vehicle driving system including an electrical differential unit and a mechanical stepped transmission unit and, more particularly, to hydraulic control over hydraulic friction engagement devices of the mechanical stepped transmission unit.
2. Description of Related Art
There is known a vehicle driving system including a mechanical stepped transmission unit in which a plurality of gear positions having different speed ratios are established in response to engaged/released states of a plurality of hydraulic friction engagement devices (see Japanese Patent Application Publication No. 11-230330 (JP 11-230330 A)). In such a vehicle driving system, the hydraulic characteristics of the hydraulic friction engagement devices at the time when the hydraulic friction engagement devices are engaged or released vary or change due to, for example, individual differences, such as variations or aging of the friction coefficients of friction materials and dimension errors of pressure receiving portions. For this reason, in JP 11-230330 A, by determining whether the ratio between an input rotation speed and an engine rotation speed has changed while the hydraulic pressure of the intended hydraulic friction engagement device is being gradually increased in a neutral state, a hydraulic pressure at which the intended hydraulic friction engagement device begins to have an engagement torque. Thus, a pack end pressure is learned. The pack end pressure is around a hydraulic pressure at which the hydraulic friction engagement device begins to have an engagement torque and is, for example, a hydraulic pressure just before the hydraulic friction engagement device begins to have an engagement torque.
Incidentally, when an electrical differential unit including a planetary gear differential mechanism coupled to at least two rotary machines and an engine is provided between the above-described mechanical stepped transmission unit and the engine (see Japanese Patent Application Publication No. 2005-344850 (JP 2005-344850 A)), it may not be possible to appropriately learn the hydraulic pressure of each of the above-described hydraulic friction engagement devices. For example, in such an electrical differential unit, if the torque of each rotary machine is controlled in a feedback manner such that an engine rotation speed, a rotation speed of an output member (which corresponds to an input rotation speed of the mechanical stepped transmission unit), and the like, become predetermined target values commensurate with a required output power, or the like, it may not be possible to appropriately detect a change in input rotation speed due to an engagement torque of each hydraulic friction engagement device. If it is not possible to appropriately learn hydraulic pressures in this way, there is a possibility that engaging and releasing control over the hydraulic friction engagement devices is impaired and, as a result, shift response deteriorates and shift shock occurs.